UC Riverside

ABSTRACT OF THE DISSERTATION

Quantification of Changes of Intensity and Attenuation Coefficient During Neural Activity Using Optical Coherence Tomography

by

Md. Monirul Hasan

Doctor of Philosophy, Graduate Program in Bioengineering

University of California, Riverside, March 2018

Dr. B. Hyle Park, Chairperson

The nervous system, which receives information through the electrical signal to coordinate and disseminate information about the body and its environment, is an assembly of neurons. Current methods of detection of neural activity are based on electrophysiology, which requires direct or near direct contact, or fluorescence-based techniques, which require the introduction of reporter molecules, raising a concern of toxicity. Optical coherence tomography(OCT) detects neural activity by taking advantage of intrinsic structural changes that accompany neural activity without any exogenous agents. The phase-resolved OCT detected 10-30 nm swelling of axon during neural activity from the functionally stimulated compound eye of the horseshoe crab(limulus) by utilizing the phase-resolved measurement. There was a decrease of backscattered intensity during propagation of action potential due to change of scattering as well. Further study on in vitro murine brain slice showed the changes of the phase (or thickness change) as well as a decrease of back-reflected intensity during propagation of action potential. Above studies with pr-OCT were performed without scanning because scanning introduced phase noise, and the desired signal was buried under noise for the signal to noise ratio of 25~30 dB. However, spectral domain OCT was used to capture 3D volumetric intensity images during the neural activity in in vitro seizure mouse model after a period of baseline data. The average intensity of the hippocampal brain slice decreased by 10~18% from baseline during neural activity. Although intensity can separate neural activity from non-neural activity with 97.27% specificity and 97.60% sensitivity, it varies from experiment to experiment. Attenuation coefficient(µ), which quantifies how strongly a sample scatters and absorbs light is a constant for a sample, can solve the problem. During neural activities, there was a 10~15% decrease of µ. For both intensity and µ based method, there was a spatial correlation between optical changes and gold standard electrophysiology. Finally, quantified OCT intensity detected a localized action potential from CA1 of the hippocampal brain slice during ripple activation and -1% intensity threshold separated activity from non-activity.